The present invention relates to negative working lithographic printing form precursors, to their use and to imagable compositions for use thereon.
The art of lithographic printing is based on the immiscibility of ink, generally an oily formulation, and water, wherein in the traditional method the ink is preferentially retained by the image or pattern area and the water or fount solution is preferentially retained by the non-image or non-pattern area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water, while the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
New types of "waterless" lithographic printing employ only an oily ink material and preferentially ink-accepting image areas and ink-repelling non-image areas on the printing form.
A generally used type of lithographic printing form precursor (by which it will be understood to mean a coated printing form prior to exposure and development) has a radiation sensitive coating applied to an aluminum substrate. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to radiation of a suitable wavelength hardens in the exposed areas. On development, the non-exposed areas of the coated composition are removed, thereby leaving the image. In contrast, positive working lithographic printing form precursors have a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble (in a developer) in the exposed areas than in the non-exposed areas. In both cases only the image area on the printing form itself is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film has typically been applied to the printing form precursor with a vacuum to ) ensure good contact. The printing form precursor is then flood exposed by a radiation source. In the case where a positive form precursor is used, the area of the film that corresponds to the image in the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating, which becomes more soluble and is removed on development.
The coatings are normally laid down as solutions in an organic solvent, which is removed by evaporation.
The radiation source has conventionally been an ultra-violet (UV) radiation source. Recently infrared (IR) radiation sources have become of interest.
In many proposals the compositions contain IR absorbers which convert IR radiation to heat, and it is the heat which is the direct cause of the solubility change in the composition. A heated body delivering heat conductively to corresponding compositions not containing IR absorbers can likewise effect the solubility change. A suitable IR radiation source is an IR laser digitally controlled to produce the required pattern of heated areas. Thus, these methods do not employ flood exposure through a film. Some of these type of compositions are suitable for advanced "Computer-To-Plate" (CTP) techniques, and also are not additionally sensitive to ultra-violet or visible radiation. These composition thus offer the advantage, over traditional photosensitive recording compositions, that they do not need to be handled in a dark room, or under ultra-violet safelighting conditions, but can simply be handled in ordinary light.
Conventionally, the imaged precursor undergoes a development step, typically by immersion in an alkaline developer such as sodium metasilicate in water, often in a dedicated plate processor, to remove the more soluble areas and yield the printing form, ready for use.
Some systems rely on adhesion of a "stripping layer" to an imaging element. After imagewise exposure, the stripping layer is peeled away, removing non-exposed areas of the imaging element. The step of peeling away the stripping layer removes the need for a separate liquid development step.
In U.S. Pat. No. 5,688,629, a photosensitive material is disclosed, for use in the production of lithographic printing plates, comprising a support, a hydrophilic layer, a photopolymerisable layer and a stripping layer. On imagewise exposure to actinic radiation, the adhesion of the photopolymerisable layer to the hydrophilic layer and/or the stripping layer is changed. Subsequent peeling away of the stripping layer leads to removal of either the exposed or unexposed parts of the photopolymerisable and hydrophilic layers, depending on the composition, to provide positive or negative lithographic printing plates.
EP-A-738932 discloses an imaging element comprising a hydrophilic surface, a hydrophobic composition containing a photosensitive acid generator and a transfer layer. After imagewise exposure of the imaging element using UV radiation, a receptor layer is laminated, either by heat or pressure, to the transfer layer, and the receptor layer is then peeled away to remove unexposed areas of the photosensitive composition.
In EP-A-559 257, recording materials are disclosed comprising a support, a porous layer of an image forming substance, a substance capable of converting radiation to heat, a thermoplastic layer and a stripping layer. Imagewise heating may be achieved using an IR laser delivered through the support. During imagewise heating, the porous layer adheres to the support and the thermoplastic layer softens, thereby penetrating the porous layer. The recording material is subsequently overall exposed to UV or short-wavelength visible radiation through the stripping layer in order to cure the thermoplastic layer. The stripping layer is peeled away, removing the unexposed areas and leaving an image of the porous layer on the support.
U.S. Pat. No. 5,768,996 discloses a lithographic printing plate formed without a separate liquid development step, and which is intended for waterless printing. It employs toner particles from photocopiers or laser printers. In this method the desired image is formed on a master paper substrate with toner, for example using a laser printer in its usual way. A layer of silicone rubber is then applied over the entire surface bearing the toner and subsequently the silicone rubber layer is "shaved" to reveal the ink-receptive toner underneath, and leave exposed toner areas, ink receptive, and silicone areas, ink repellent.
An object of the present invention is to provide a simple negative-working thermally imagable composition suitable for a lithographic printing form precursor. Objects of preferred embodiments of the present invention include: providing such a precursor which can be water or fount solution developed, thus including on-press, during printing; providing a composition and precursor which employs toner particles as radiation absorber; and providing a composition which can be laid down as a coating from an aqueous solution, thus avoiding the need to remove an organic solvent by evaporation.